Voltage‐gated ionic currents in an identified modulatory cell type controlling molluscan feeding

An important modulatory cell type, found in all molluscan feeding networks, was investigated using two‐electrode voltage‐ and current‐clamp methods. In the cerebral giant cells of Lymnaea, a transient inward Na+ current was identified with activation at −58 ± 2 mV. It was sensitive to tetrodotoxin only in high concentrations (≈ 50% block at 100 µm), a characteristic of Na+ channels in many molluscan neurons. A much smaller low‐threshold persistent Na+ current (activation at < −90 mV) was also identified. Two purely voltage‐sensitive outward K+ currents were also found: (i) a transient A‐current type which was activated at −59 ± 4 mV and blocked by 4‐aminopyridine; (ii) a sustained tetraethylammonium‐sensitive delayed rectifier current which was activated at −47 ± 2 mV. There was also evidence that a third, Ca2+‐activated, K+ channel made a contribution to the total outward current. No inwardly rectifying currents were found. Two Ca2+ currents were characterized: (i) a transient low‐voltage (−65 ± 2 mV) activated T‐type current, which was blocked in NiCl2 (2 mm) and was completely inactivated at ≈ −50 mV; (ii) A sustained high voltage (−40 ± 1 mV) activated current, which was blocked in CdCl2 (100 µm) but not in ω‐conotoxin GVIA (10 µm), ω‐agatoxin IVA (500 nm) or nifedipine (10 µm). This current was enhanced in Ba2+ saline. Current‐clamp experiments revealed how these different current types could define the membrane potential and firing properties of the cerebral giant cells, which are important in shaping the wide‐acting modulatory influence of this neuron on the rest of the feeding network.